The direct reduced iron producing method has been known as a method for directly reducing an iron oxide source such as iron ore and iron oxide (which may be hereinafter referred to as iron oxide-containing material), by using a carbonaceous reducing agent (carbonaceous material) such as coal and a reducing gas so as to obtain reduced iron. The direct reduced iron producing method is based on such a procedure as charging a raw material mixture including the iron oxide-containing material and the carbonaceous reducing agent onto the hearth of a moving hearth-type thermal reduction furnace (for example, rotary hearth furnace), heating the raw material mixture with the heat from a burner and radiation heat while the raw material mixture is moved in the furnace so as to reduce the iron oxide included in the raw material mixture by the carbonaceous reducing agent, carburizing and melting the metallic iron (reduced iron) thus obtained, coalescing the molten metallic iron to granules while separating it from the subgenerated slag, and cooling and solidifying the molten metallic iron so as to obtain granular metallic iron (reduced iron).
The direct reduced iron producing method does not require a large scale facility such as blast furnace and has high flexibility with regards to resources for example, this method makes it unnecessary to use coke, therefore recently has been vigorously studied for commercial application. However, the direct reduced iron producing method has various problems to be solved in order to be applied on an industrial scale, including the stability of operation, safety, economy and quality of the granular metallic iron (product).
The granular metallic iron produced by the direct reduced iron producing method is sent to an existing steel making facility such as electric furnace or converter, and is used as the iron source. Therefore, with respect to the quality of the granular metallic iron, it is required to decrease the sulfur content in the granular metallic iron (may be hereinafter referred to as S content) to as low a level as possible. It is also desirable that the carbon content in the granular metallic iron (may be hereinafter referred to as C content) is high within a reasonable range, in order to broaden the applicability of the granular metallic iron as the iron source.
The inventors of the present application previously proposed a technology disclosed in Patent Document 1, which increases the purity of granular metallic iron so as to improve the quality of the granular metallic iron. Patent Document 1 discloses a method of increasing the purity of the granular metallic iron, which prevents the metallic iron from being oxidized again in a zone from the last stage of reduction to the completion of carburization and melting by controlling the reducing degree of the atmospheric gas in the vicinity of the compacts during carburizing and melting to a proper level.
Patent Document 1 also describes a technology to decrease the sulfur content in the granular metallic iron. Specifically, such a method of decreasing the sulfur content is disclosed that is based on controlling the basicity of the slag which is a byproduct generated when melting the metallic iron.
The inventors of the present application also previously proposed a technology described in Patent Document 2, besides that of Patent Document 1, which decreases the sulfur content in the granular metallic iron. Patent Document 2 discloses a method of decreasing the sulfur content in the granular metallic iron by controlling the basicity of the slag-forming component, that is determined from the composition of the raw material mixture, and controlling the MgO content in the slag-forming component.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-279315    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-285399